Scanners and copiers are well-known office machines that provide valuable office functions both in the workplace and at home. One important component of these scanners and copiers is the image processing that automatically filters noise (unwanted information) from the scanned document.
Two-sided documents, i.e., documents having text and/or pictorial content on both sides of the paper, present challenges for producing quality copies of the original documents. When two-sided documents are scanned in a copy machine or a scanner, visual noise may appear in the copies that was not present on the scanned surfaces of the original documents. The visual noise may be the result of digitally captured text and/or pictorial content printed on the opposite side of a scanned surface commonly referred to as bleed-through. Bleed-through is more prevalent for copies of documents having a white or very light color background. In addition, the thickness of the scanned documents may increase the intensity of the bleed through effect, since thinner paper is more transparent than thicker paper. Moreover, when scanning a paper document, the paper is generally not captured as exactly white as a result of inaccurate calibration of the scanner with respect to each paper type. The combination of the non-white background and bleed-through both reduce visual quality of the scanned documents, especially when printing them, since slightly off-white color is typically rendered as scattered dots by printers.
Certain known background noise removal techniques consist of estimating the background color of the scanned-in document by analysis of local or global tone-statistics of the document image, and then applying a local or global tone-mapping which maps the estimated background color or lighter tones to pure white.
FIG. 1A illustrates the steps of one type of prior art scanning or copier system including scanning 10, image processing 11, rendering 12, and displaying or printing 13 a document image. Frequently, this type of system uses a type of processing technique referred to as “single pass” since typically all operations are performed within the system (e.g., copier) in a single irreversible processing pass. A system designed using the “single pass” technique generally only allows for a small amount of buffered image data for processing. The entire image is not stored and all processing is performed within the system such that there is no need for image compression, transmission, decompression, or off-line computing. Processing is often performed cumulatively and then processed data is immediately printed. As shown in FIG. 1A, background noise removal is performed during image processing. To date, this background noise removal technique has been irreversible in the “single pass” type system. In other words, once the background noise removal technique has been performed the original image data of the document is no longer available due to the single pass processing technique which can potentially cause a loss of visually important information. In some cases background noise removal may be undesirable if the type of document image does not lend itself to typical background noise removal processes and removal results are typically poor. However, according to this system, if the background noise removal technique does not perform satisfactory, the user does not have the option of regaining or using the visual information-lost during the background removal process.
FIG. 1B shows the steps of another type of known scanning or copier system which can be formed by using an add-on background removal function with a pre-existing scan system. According to this type of system, image data is stored and available for performing background removal operations, however, the background removal operations are performed inefficiently within the system. Specifically, a document image is scanned 14 and processed 15 to enhance the image. After scanning and processing, the processed image data is stored 16. Background noise removal 17 is performed by accessing the stored image and performing all of the necessary statistical operations to derive a background noise removal function. These operations often require highly computational and complex statistical image data analysis. Hence, each time background noise removal is performed in the system shown in FIG. 1B, these highly computational steps are repeated. Often, the analysis performed during background noise removal is similar to or the same as the analysis performed during image processing step 15, however, according to this type of prior art processing technique these highly computational analysis and processing steps are repeated since all of the background removal is performed after the image has been stored and the prior analysis results are not made available for use during background noise removal. Consequently, this type of system performs background noise removal in an inefficient manner.
Hence a need exists for an efficient system and method of background noise removal of a scanned document that also provides a user with the ability to reverse and/or control the background noise removal process after scanning the document.